A conventional bipolar transistor is known for the fact that the electric current gain is small in a very low electric-current level, large in a middle electric-current level and small again due to a high level injection effect in a high electric-current level. For example, it is described as a typical example in pages 142-143 and FIG. 7 of non-patent document 1 that the electric current gain is 35 at the collector electric current of 100 pA, and increases by approximately one order of magnitude according to the increase of the collector electric current to reach the maximum gain of 400 at about 100 μA, and then decreases for larger collector electric current.
On the other hand, a graft base transistor structure is conventionally known for achieving a small external base electric resistance and improving the high frequency characteristics. As illustrated in FIG. 6.12(A) in page 106 of non-patent document 2, this structure is a structure in which a high-impurity-concentration base region (graft base 127) is extended from an edge surface of an emitter 130 laterally to under the emitter and buried therein. In this case, when the high-impurity-concentration base region is separated from the emitter, the electric resistance from the region to an intrinsic base 121 under the center of the emitter becomes high and therefore this is not advantageous. FIG. 1 illustrates a sectional view of a transistor having the graft base structure extracted from FIG. 6.12(A) of non-patent document 2. In FIG. 1, the high-impurity-concentration base region 127 is extended with a length of 0.35 μm from the edge of the emitter 130 to under the emitter and buried therein. In FIG. 1, the reference numeral 110 is the collector, the reference numeral 121 is the intrinsic base, the reference numeral 127 is the graft base and the reference numeral 130 is the emitter. These reference numerals are overwritten in FIG. 6.12(A).
This graft base transistor is not developed for improving the electric current dependence of the electric current gain. Since the graft base transistor is developed for improving the high frequency characteristics, the operating range mainly covers large electric current and less emphasis is put on the very low electric-current level.
Thus, the structure in which the high-impurity-concentration part of the high-impurity-concentration base region of the graft base transistor has contact with the emitter increases the leakage electric current between the emitter and the base. This may lead the decrease of the electric current gain in the very low electric-current level near the value of the leakage electric current. Therefore, the range of the collector electric current, for which the gain variation is small, is unfavorably narrowed. In addition, the breakdown voltage between the base and the emitter becomes small and the application voltage range of the transistor is narrowed, which is not preferable.    [Non-patent Document 1]    S. M. Sze, “Physics of Semiconductor Devices”, second edition, John Wiley and Sons.    [Non-patent Document 2]    L. Treitinger, M. Miura-Mattausch, “Ultra-Fast Silicon Bipolar Technology”, Springer-Verlag